When duplex scanning (double sided) the problem of aligning the image from the front side with the image of the rear side becomes a problem. Longitudinal misalignment of these images manifests itself as a black border along the leading edge of the document images. Lateral misalignment (parallel to the direction of direction of document travel) manifests itself as borders on the left and right edges of the front and rear images respectively.
Currently available duplex scanners generally have no provision for lateral alignment or require a mechanical alignment when the scanning cameras are initially installed. Such an approach requires additional mechanical expense and the adjustments can be difficult and time consuming, especially during field replacement of the scanning cameras.
Most scanners have at least one cropping window which allows a large format scanner to reduce the amount of data generated when scanning a document smaller than the full format. For example, a 11.times.17 inch scanner could crop an image down to 8.5.times.11 if necessary.
Often platen scanners do two scans of a document, the first is a pre-scan to adjust the cropping window, and the second scan is used to capture the image. This is very advantageous because all borders are removed from the image.
However, in a high speed rotary scanner, it is not possible to perform two scans to establish a cropping window. Because the document is moving in the longitudinal direction, it is feasible to use a low resolution sensor to pre-scan to provide cropping of the leading and trailing edges of the document. Because the document is not moving in the lateral direction, it is expensive to automate the lateral cropping and for this reason, initial alignment becomes more important.